The present invention relates generally to quality control inspection of nuclear reactor fuel rods, and more particularly, to an apparatus and method for inspecting TIG (tungsten inert gas) welds of an end plug on a sealed fuel rod.
A typical nuclear fuel rod includes an elongated cladding tube containing nuclear fuel pellets, a bottom end plug girth welded to the bottom of the cladding tube, and a top end plug girth welded to the top of the cladding tube. The top end plug usually contains an axial bore through which the interior of the cladding tube is pressurized with a gas (such as helium). The axial bore is then closed with a seal weld. The girth and seal welds are typically TIG welds. Quality control inspection of the manufactured fuel rod has included inspection of the top end plug's girth and seal welds, and the bottom end plug's girth weld.
In a conventional X-ray weld inspection technique, an X-ray film was developed for a weld area and was subjectively evaluated by an inspector. Relatively heavy shielding was required to minimize radiation exposure to personnel. The X-ray system detected tungsten inclusions in the top end plug's seal weld. Since the seal weld was made under pressure, a higher potential was required for the welding electrode to overcome the dielectric of the pressurized gas, and tungsten from the TIG welding tip could be ejected into the weldment. Tungsten inclusions were less of a problem with the unpressurized girth welds. The X-ray system also detected voids in the girth and seal welds. By a "void" is meant an area devoid of weldment which should have such weldment. It includes a cavity within a weldment as well as a weldment which is lacking in desired thickness.
Conventional ultrasonic flaw detection techniques have been applied to material and weld inspection, typically in non-nuclear fuel rod areas. Existing ultrasonic inspection systems have included the use of multiple transducers for better void detection, the capability of automatic testing, and the use of flaw alarm monitors. Pipes have been tested in transit tanks according to the immersion technique, with spiral translatory motion of the pipes. The entrance and exit openings of the tank were sealed in such a way that, during the test run, the pipes remained submerged to a sufficient depth. Canning tubes for nuclear reactor fuel elements have been tested by the immersion method using a long tank with completely immersed pipes and traveling probes, or using a transit tank through which the pipe is fed in spiral motion, with the probes remaining stationary. Ultrasonic weld inspection systems detect the material/air boundary of voids (either the thin wall or cavity type), but cannot reliably detect tungsten inclusions (which give a material/material boundary) which can arise from TIG seal welding. The presence of a precalculated amount of tungsten is a weld defect which must be tested for by any nuclear fuel rod end plug weld inspection apparatus.